The present invention relates generally to improved formulations for cooling water, and to the in-service treatment and maintenance of systems using relatively large quantities of cooling water, and more specifically to such a formulation which may be utilized to both restore system performance through removal of scale on an in-service basis, while at the same time being designed to maintain the system in proper condition through subsequent prevention or substantial elimination of build-up or redeposition of additional scale. The present invention is an improvement over that disclosed and claimed in the prior U.S. Patent of Bruce D. Busch, No. 4,279,768, granted July 21, 1981, entitled "SERVICE DESCALER FOR AQUEOUS SYSTEMS", and assigned to the same assignee as the present invention.
In the operation of systems utilizing cooling water, quantities of water as required are introduced and utilized at a rate dependent upon the service requirements of the installation. As introduced, make-up water universally contains a quantity of impurities and/or contaminants which, unless treated, contribute to a constant build-up of scale on the heat transfer surfaces or working surfaces of the system. Normal operation of these systems results in the consumption of reasonably large quantities of water, and as water is lost from the system, normally through evaporation, levels of concentration of these contaminants and/or impurities increase. These contaminants and/or impurities are normally those same elements or compounds which commonly contribute to water hardness, specifically certain calcium salts along with certain quantities of iron oxides. As has been recognized in the past, scale build-up on heat transfer surfaces in the aforementioned equipment causes ongoing expense to the owner as well as possible exposure to dangerous conditions for those persons located near such equipment. Scale build-up increases thermal impedance and thus decreases the rate of heat transfer through the surfaces. This contributes to an overall loss of efficiency of the system as well as increased energy consumption and costs. Scaled heat transfer surfaces ar susceptible to corrosion beneath the scaled area since corrosion control agents are unable to effectively contact the metal surfaces. Furthermore, scale acts as an insulative barrier, resulting in inefficient heat transfer. This condition leads to both increased energy costs and decreased system efficiency. System component failures can occur due to the higher than normal temperatures being required at the heat transfer surfaces in order to maintain normal operation and/or operation requiring extraordinary performance. Thus, clean scale-free heat transfer surfaces represent a desirable condition in cooling water systems.
In the prior U.S. Pat. No. 4,279,768 mentioned above, an in-service descaler is disclosed containing a number of the components present in the present formulation. The present invention constitutes an optimization of those disclosed in U.S. Pat. No. 4,279,768, and furthermore incorporates other components which contribute to the effectiveness of the present formulation for cooling water treatment while the system remains on-line. The primary components of most water impurity based scales in the equipment mentioned above are calcium salts (e.g. calcium carbonate, calcium orthophosphate, calcium sulfate and calcium bicarbonate) as well as iron oxides. These compounds are present in most domestic and commercial water supplies, being normally found in surface and in subterranean water sources.
The present material is particularly adapted for use in reducing scale, as well as inhibiting corrosion in open recirculating cooling water systems. Furthermore, the formulation of the present invention is free of chromate, and the formulations eliminate the requirement for the addition of quantities of sulfuric acid. Corrosion control is provided with this system, for both ferrous and non-ferrous (cuprous) metals.
The presently commercially available and widely utilized cooling water scale prevention includes formulations utilizing chelants, polymeric conditioners and certain gluconates as sequestering agents.
The present system utilizes chelants such as tetrasodium ethylene diamine tetraacetic acid (EDTA) and trisodium nitrilotriacetate (NTA) to stoichiometrically sequester calcium and iron ions. Stoichiometric treatments require significant concentrations to prevent scale formation, particularly when the water source contains substantial quantities of calcium and/or iron ions. Gluconates and polyelectrolytes are normally threshold treatments. Threshold treatments are generally effective in controlling large numbers of ions per unit of treatment, and the economy of using such threshold treatments is readily apparent. Specific modified formulations in use today are disclosed in U.S. Pat. No. 4,279,768.
In addition to the formulations of U.S. Pat. No. 4,279,768, scale removal has been accomplished by descaling the thermal energy transfer surfaces of cooling towers and the like with acid treatments. Organic acids such as citric and sulfamic or mineral acids such as sulfuric and/or hydrochloric are frequently used to remove scale of the type mentioned above. However, in acid descaling, the system must normally be shut down, drained, acid cleaned, rinsed, drained for a second time and retreated. Acid descaling thus requires either waiting for a convenient time to shut down operations o possibly interrupting production schedules on an emergency basis to achieve cleaning. Either alternative is known to be costly, since draining of the system is often required, and an added cost is the replenishing of chemical treatment once the system is clean.